1. Field of the Invention
This invention relates to a method of measuring humidity of an oxygen-containing gas by using an electrochemical cell of an oxygen-ion conductive electrolyte on which a pair of electrodes are placed.
2. Description of the Prior Art
In a humidity measurement device using the electrochemical cell, there have been devices known in Japanese Patent Provisional Publication Nos. 60-222761, 62-150151 and 62-150152.
In the Japanese Patent Provisional Publication No. 60-222761, partial pressure of an oxygen component in moisture-laden gas is measured with the function of limit currents, each value of which is obtained before and after removing aqueous vapor component from the gas.
The Japanese Patent Provisional Publication No. 62-150151 teaches that the limit currents due to gas diffusion are obtained depending on an oxygen density and an aqueous density in the gas. The difference between values of the limit currents leads to humidity measurement.
Further, the Japanese Provisional Publication Pat. No. 62-150152 suggests that a first limit current due to an oxygen density and a second limit current due to an aqueous density are obtained. Then, a third limit current based on a moisture-laden gas is obtained. The humidity is measured based on the difference between the third limit current and, at least, one of the first and second limit currents.
In each device according to the Japanese Patent Provisional Publication Nos. 60-222761, 62-150151 and 62-150152, however, a drying device is required to desiccate the gas to remove aqueous vapor. It is, simultaneously, necessary for the prior devices to measure the limit current. Thus necessitates a large scale measurement system as a whole, and at the same time, resulting in a slow response to operation of the humidity measurement.
On the other hand, porous electrodes or porous oxygen-ion diffusion limiting means is provided in the solid electrolyte, variation of the porosity inevitably occurs according to individual products. The same is true with thickness and density of the solid electrolyte. This causes to establish variation in diffusion limit currents due to conductivity difference of the electrolyte and various gas diffusions under a certain voltage applied across the electrodes, thus resulting in discordance with humidity of the gas. Further, replacement of parts and elements of the device becomes difficult in ensuring the humidity measurement.
Moreover, the electrolysis between the electrode and the solid electrolyte tends to deteriorate an interface therebetween with the moisture so as to gradually alter the characteristics of the electrochemical cell, thus making it impossible to ensure a stable humidity measurement for an extended period of time.
Therefore, the invention has its object to provide a method of measuring humidity, which is capable of eliminating all the drawbacks mentioned above.
It is an object of the invention to provide a method of measuring humidity which is capable of maintaining measurement of humidity level to be unchanged for an extended period of time with high accuracy.
It is an object of the invention to provide a method of quickly and easily measuring humidity.